This invention relates to an integrated apparatus and process for vacuum forming a thermoplastic resin sheet to provide a preform followed by reaction injection molding of a thermoset resin against the preform to provide a shaped polymeric laminate article.
In thermoforming, a thermoformable polymeric sheet, e.g., one fabricated from a thermoplastic resin such as poly(methyl methacrylate) (PMMA), is heated to thermoforming temperature and with the use of positive or negative pressure (vacuum), is then conformed to the shaping surfaces of a mold. When a vacuum is used to draw the heat-softened sheet against the shaping surfaces of the mold, the process is referred to as vacuum forming.
Reaction injection molding involves a one-shot injection of a flowable, substantially homogeneous mixture of polymerizable components into a sealed mold where rapid polymerization occurs resulting in a molded resin product which may be of thermoplastic, elastomeric or thermosetting variety depending on the relative number of crosslinks which may be present. In a typical reaction injection molding process, the viscosity of the resin-forming composition introduced into the mold cavity ranges from about 50 cps to about 10,000 cps, preferably from about 500 cps to about 5000 cps. Mold temperatures generally vary from about ambient to about 150.degree. C. and mold cavity pressures generally range from about 30 psig to about 200 psig, preferably from about 50 psig to about 100 psig.
In a reaction injection molding process, the polymerization reaction takes place fairly rapidly, e.g., in less than about 5 minutes and frequently in less than about 2 minutes For further details regarding known types of reaction injection molding apparatus and processes, reference may be made to Sweeney, "Introduction to Reaction Injection Molding", Technomics, Inc. (1979) as well as to the considerable patent literature on the subject.
The reaction injection molding of a thermoset resin against a thermoplastic resin preform to provide a shaped polymeric laminate article is a known technique.
As disclosed in U.S. Pat. Nos. 4,053,545, 4,784,904, 4,822,652, 4,844,944 and UK Patent Application GB 2 148 786 A, a laminate article such as a bathtub or shower tray is manufactured by placing a preformed shell or liner, e.g., a thermoformed acrylic sheet, within the cavity of a reaction injection mold and thereafter injecting a flowable thermoset resin foam-forming composition, e.g., one providing a rigid, dense microcellular polyester or polyurethane resin foam, into the mold. Following curing of the foam, a laminated product is obtained in which the thermoplastic resin preform is backed with a reinforcing layer of foamed thermoset resin.
Until now, it has been the practice to form the preform in an entirely separate molding operation employing conventional thermoforming apparatus and techniques So, e.g., in thermoforming an acrylic sheet into a bathtub shell, the sheet is heated to thermoforming temperature and thereafter conformed to the shaping surfaces of a female mold. Although it is possible to fully form the acrylic sheet against shaping surfaces of a male mold unit and thereafter clamp the female mold unit in place to provide a cavity corresponding to the shape of the polyurethane foam component to be formed therein, such a procedure is impractical since the acrylic sheet will fail to undergo any significant amount of stretching in the area in which it makes contact with the flat horizontal surface of the male mold, i.e., the section of the male mold corresponding to the floor, or bottom, of the bathtub shell. Were the bathtub shell, or preform, to be formed in this manner, the average thickness of the side walls would be a good deal less than that of the bottom of the shell, as a result of which the side walls would be readily susceptible to breakage or other damage Of course, a thicker acrylic sheet could be utilized in fabricating the preform by the foregoing procedure but this would involve a significant economic penalty since the bottom of the preform would contain more resin than required to meet any structural or functional standard.
Accordingly, in the manufacture of thermoplastic/thermoset resin laminates in which the thermoset resin component is applied to the lower surface of the thermoplastic resin preform component, it has been considered necessary to mold the preform entirely against the shaping surface of the female mold cavity of a thermoforming apparatus, remove the thermoformed preform from the mold cavity, transfer the preform to the cavity of a reaction injection molding apparatus and reaction injection mold a rigid thermoset foam reinforcing layer against the preform, i.e., the manufacturing procedures of U.S. Pat. Nos. 4,053,545, 4,784,904, 4,822,652 and 4,844,944 and UK Patent Application GB 2 148 786 A referred to above.